What I don't get is why they can do this with ghost electricity from wifi, but not ghost electricity from other appliances also such as HDTVs, Blu-Ray plays, ...and other appliances that emit ghost electricity.

I think by ghost electricity you're referring to phantom loads which refers to devices that draw power even when they're off. Think about a TV for example. When you turn it off you're not really turning it, but just putting it in standby mode. There has to be a minimal amount of circuitry powered up to be able to turn it on using the remote control. It's a small amount, but still not zero.

This works be detecting the electrical energy of the Wi-Fi radio waves, not the actual content of the data that can be sent. It is a passive method that requires no connection and does not create any more or less load on the system. The waves are in the air regardless if something is picking them up or not.

Think about a crystal radio like the one kids make in high school. It has no power source but still is able to create enough current to power a small earphone to hear a radio broadcast. That energy came directly from the radio wave.

In theory you could do this with other frequencies besides Wi-Fi, but Wi-Fi is a much stronger signal compared to broadcast TV and radio, which is why th...(continues)